1. Field of the Invention
The present invention relates to a porous landscape pebble laid mainly on a ground, a building's roof, etc. to provide an excellent effect as a countermeasure against a heat island for a median strip of a road, a green zone or paving stone of a park, or gardening, and a method of manufacturing the same.
2. Description of the Related Art
Such a landscape pebble having an effect as a heat island countermeasure is required to be porous and capable of absorbing and holding a great quantity of water. The porous landscape pebble capable of holding much water is effectively cooled by high heat of evaporation of the held water. For this reason, spreading the porous landscape pebble has an effect as the heat island countermeasure. In summer, a heat island phenomenon occurs in many places to cause negative impacts on the living environment. For instance, in the cities when buildings are in close formation, because asphalt or concrete of a building roof, a park, a garden, a road, etc. is heated by the sun to cause the heat island phenomenon. In order to cope with the use of temperature caused by the heat island phenomenon to thus make a living space comfortable, air cooling is performed inside of the building. However, since the air cooling discharges heat than the quantity of heat to be cooled, this is responsible for further increasing temperature outside the building and thus raising temperature of the city. In this respect, a method of rooting up the heat island phenomenon is required. The porous landscape pebble is characterized by a capability to remove the heat island phenomenon and make the landscape beautiful. However, the pebbles which are produced by crushing ordinary stones are impossible to remove the heat island phenomenon. The landscape pebble for removing the heat island phenomenon requires all of the following (1) to (4) physical properties.
(1) To be porous and to contain a great quantity of water,
(2) To be lightweight,
(3) To be strong enough to resist destruction even when walked thereon, and
(4) To be inexpensive.
However excellent the landscape pebble may be in properties, its wide construction area dose not allow it to be practically used as long as the heat island phenomenon cannot be removed inexpensively and economically. Further, the landscape pebble is cooled by use of the evaporation heat of water. Hence, without preservation of a great quantity of water, it is impossible to effectively remove the heat island phenomenon for a long time. In addition, without being lightweight, it is impossible to spread the landscape pebble on the building roof, etc. Moreover, because the landscape pebble is mostly used for the place on which people walk, the landscape pebble is remarkably restricted to its application when it is so weak that is destroyed by walked upon.
Meanwhile, currently, the fuel used in a power plant, etc. is switched from heavy oil to coal, so that a large quantity of coal is used for the production of electrical power, etc. The coal is burnt after crushed and becomes fly ash. Therefore, up to now, a tremendous quantity of fly ash has been generated as a waste. The fly ash is of very fine inorganic particles. The fly ash is disposed by filled in cement, and its scrapped amount is too enormous to be developed in application and used effectively. For this reason, there has no choice but to dispose of the fly ash by a method such as landfill, etc. However, the fly ash is never preferred as a soil of landfill.
In order to make effective use of the fly ash, there has been developed a technique where the fly ash is sintered and used as an aggregate (see Japanese Patent Laid-Open Nos. 1997-156998, 1994-9251, and 1997-30857).
Japanese Patent Laid-Open No. 1997-156998 adds an inorganic binder to the fly ash and sinters the fly ash. The fly ash essentially consists of silicon dioxide and aluminum oxide, and cannot be sintered at a low temperature because the component corresponding to a fusing agent of a low melting point is small. Further, the fly ash has a narrow range of firing temperature, so that it cannot be sintered when the firing temperature is too low. To the contrary, when the firing temperature is too high, the whole is fused and sintered. In order to remove this problem, the technology described in Reference Document 1 adds a clay mineral, preferably bentonite, as the inorganic binder to the fly ash in a range of 3 to 5 wt % before firing. The fired formation can be sintered at a relatively low temperature. However, the sintered material has a drawback that the cost of raw material is increased because the inorganic binder is added to the fly ash. This is because the inorganic binder, i.e., bentonite is expensive in comparison with the fly ash as a waste. Further, the addition of the inorganic binder is responsible for decrease in the water absorption rate of a sintered body. This is because the inorganic binder is filled into pores in order to bind the fly ash, and thereby reducing porosity. Thus, when an aggregate is manufactured by this method, its manufacturing cost is raised. Nevertheless, there is a disadvantage that a characteristic of holding water, which is important for landscape pebble, is deteriorated.
Japanese Patent Laid-Open No. 1994-9251 discloses a method of manufacturing an artificial lightweight aggregate, comprising adding heavy oil ash having much unburned carbon to fly ash having little unburned carbon, pelletizing the fly ash and firing the pelletized fly ash. The heavy oil ash is mixed with the fly ash, so that the artificial lightweight aggregate is sufficiently sintered by the unburned carbon of the heavy oil ash. This method makes use of both of the fly ash and the heavy oil ash, as well as the heavy oil ash having much unburned carbon. For this reason, there is a drawback that it is troublesome to manage two kinds of raw materials. Furthermore, it is difficult to enhance the absorptive property and simultaneously extend a time capable of holding absorbed water according to the unburned carbon added through the heavy oil ash. This is because it is difficult to enhance porosity by forming micro pores in the aggregate sintered with the unburned carbon contained in the heavy oil ash. The heavy oil ash is added to the fly ash in place of pulverized coal, wherein the added carbon in the state of unburned carbon of the pulverized coal and the heavy oil ash is difficult to form the micro pores inside the aggregate. This is because the carbon is added and sintered to the outside of the fly ash, so that the pores caused by burning down the carbon are formed between the outsides of the fly ashes. Hence, this method also has a drawback in that, in spite of complicated method of manufacturing the aggregate by adding a specified quantity of heavy oil ash to the fly ash, the manufactured aggregate cannot be endowed with an excellent property.
Further, Japanese Patent Laid-Open No. 1997-30857 discloses a method of manufacturing a sintered body by sintering fly ash containing unburned carbon. In this document, sintering is performed by use of special fly ash having less unburned carbon, because the unburned carbon contained in the fly ash deteriorates a physical property of the sintered body. The unburned carbon of the fly ash is burnt to form pores, thus having influence on the physical property of the sintered body. Further, the unburned carbon is accompanied with air on porous surfaces, and the air is inflated and stripped on heating, so that a crack is generated in the sintered body. In addition, the oxidation atmosphere during sintering becomes a reduction atmosphere due to the burning of the unburned carbon, thus causing sintering defects. For these reasons, using the fly ash having less unburned carbon is described.
The present inventor makes effective use of unburned carbon which has been regarded to have an adverse effect on the sintered body of the fly ash, thereby succeeding in developing a landscape pebble capable of effectively removing the heat island phenomenon. Therefore, a main object of the present invention is to provide a landscape pebble and a method of manufacturing the same, capable of mass-producing the pebble at a low cost by increasing the porosity thereof to thus increase the water absorption rate in order to improve the water holding characteristic, and moreover by making the pores in a very fine state, and further by causing the whole to be lightweight and to have the sufficient strength in order to spread the landscape pebbles on the roofs etc., and additionally by using the fly ash containing the unburned carbon which is considered to be difficult to use effectively.